Hi Here, it means: legitimate, acceptable, within the rules It is quite an old metaphor (17th century) and it refers to card-playing. A dishonest card-player might move his hand beneath the card-playing table (the board) so as to switch his cards for some false ones that he has hidden. If you are satisfied that nothing like that has happened then everything is above board The writer was wondering whether the performance art was part of the gallery's program or whether it was some rogue activists who were performing unofficially, without permission Dave
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